The present invention relates to a hologram-recorded medium and a process for the fabrication of the same, and more particularly to a process for the fabrication of a computer-generated hologram in which an optical pattern is formed on a given recording surface by computer-aided computation and a hologram-recorded medium obtained by the same.
In recent years, coherent light has been easily obtainable by use of lasers, and holograms have been widely commercialized as well. Especially for notes and credit cards, the formation of holograms on portions of their media has become popular for anti-counterfeiting purposes.
Today's commercially available holograms are obtained by recording original images on media in form of interference fringes, using optical techniques. That is, an object that forms an original image is first provided. Then, light from this object and reference light are guided through an optical system such as a lens onto a recording surface with a photosensitive agent coated thereon to form interference fringes on the recording surface. Although this optical technique requires an optical system of some considerable precision for the purpose of obtaining sharp reconstructed images, it is the most straightforward method for obtaining holograms and so becomes most widespread in industry.
On the other hand, techniques for forming interference fringes on a recording surface by computations using a computer for the fabrication of holograms, too, have been known to those skilled in the art. A hologram fabricated by such techniques is generally called a computer-generated hologram (CGH for short) or referred to simply as a computer hologram. This computer hologram is obtained by computer simulation of a process of generation of optical interference fringes, which process is all performed through computer-aided computations. Once image data on an interference fringe pattern have been obtained by such computations, physical interference fringes are formed on an actual medium. A specific technique has already been put to practical use, in which image data on a computer-generated interference fringe pattern are given to an electron beam lithographic system, so that the data are scanned by electron beams on a medium thereby forming physical interference fringes on the medium.
While keeping pace with recent developments of computer graphics, computer-aided processing of various images is being generalized in the printing industry. For the original images to be recorded in holograms, too, it is thus convenient to provide them in the form of image data. In consideration of such demands, techniques for generating computer holograms are of growing importance, and expected to take over optical hologram fabrication methods at some future time.
As already mentioned, micro-characters by printing are now often used as anti-counterfeiting means for notes, credit cards, etc. The micro-characters, because of being little perceivable by the naked eyes, are effective for anti-counterfeiting purposes. However, recent improvements in the performance of copiers enable general printed micro-characters to be copied with some precision. To utilize micro-characters as anti-counterfeiting means, something new is in need.
For practical solutions to such technical challenges, for instance, Utility Model No. 2582847 discloses a method for recording micro-characters having a maximum size of 300 μm or less in the form of a diffraction grating pattern. Such micro-characters recorded as the diffraction grating pattern cannot be copied on current ordinary copiers and so are very effective for prevention of counterfeiting by copiers. It is here understood that such recorded micro-characters of 300 μm or less are authenticated on an enlarged scale under loupes or the like because they cannot visually be perceived. Conversely speaking, the use of loupes, microscopes or the like will enable any person to check the content of authenticating information recorded in the form of micro-characters. Given recently developed, relatively inexpensive, easy-to-obtain devices capable of recording fine diffraction gratings, there is a possibility that the content of authenticating information recorded as micro-characters may be decoded, and counterfeited by a diffraction grating recorder.
Thus, authenticating information recorded by use of diffraction gratings, because of being recorded directly on a recording surface, is vulnerable to copying. On the other hand, the recording of authenticating information in the form of a hologram pattern is superior in view of prevention of counterfeiting, because an interference fringe pattern is simply recorded on a recording surface; the authenticating information itself is not recorded directly on the recording surface. For instance, JP-A 11-21793 discloses an optical hologram fabrication process wherein a real original image comprising characters of normal size is recorded by optical reduction processing as a micro-character hologram pattern.
Of course, it is in principle possible to fabricate such micro-characters using a computer hologram methodology. However, there is still a grave problem with commercialization of computer holograms, that is, excessive computation loads on computers. To obtain reconstructed images of high precision, an original image must be processed as a set of a multiplicity of micro-sources of light. It is then necessary to compute, for each position on a recording surface, the intensity of object light coming from all the micro-sources of light and reference light. To fabricate a computer hologram for an original image such as one comprising micro-characters of visually unperceivable size, computation must be performed with very high precision, imposing some considerable computation loads on the computer used. Although it is prima facie possible to fabricate a computer hologram equivalent in quality to an optical hologram by implementing long-term computation using a super-fast computer having improved computing power, yet such a fabrication process cannot be utilized for commercial purposes. Furthermore, the capacity of image data having information on interference fringe patterns fabricated by computation becomes massive, and operational loads on the formation of interference fringes on a physical recording medium using an electron beam lithographic system becomes massive as well.